Book Reference : Pages To recap the nature of the magnetic field around a bar magnet & the Earth 2.To understand the nature of the magnetic field around a current carrying wire 3.To be able to calculate the magnitude & direction of the force on a current carrying wire in a magnetic field

Definition : A magnetic field is a force field which surrounds either a magnet or a wire carrying an electric current and will act upon, without contact, another magnet or current carrying wire Like the other fields we have studied we represent magnetic fields diagrammatically using field lines or lines of magnetic flux Plotting Compass

We name the ends of a magnet “the poles”. (North and & South). More correctly they should be referred to as the “North seeking pole” and “South seeking pole” The arrows on a magnetic field line represent the path which a “tiny free north pole” would take Like poles repel each other Unlike poles attract each other

The Earth has a magnetic field just like a giant magnet. The geographic North pole has a South magnetic pole associated with it (Since a north seeking pole of a magnet will point towards it) Earth Dipole

Watch terminology!!! At the geographic North pole there is a magnetic pole which we can refer to as “Magnetic North”. However it is a South pole! Watch the field lines!

Wires carrying a magnetic current produce a magnetic field “Maxwell’s corkscrew rule” can be used to establish the direction of this field. Note the current direction is the direction of “conventional current” positive to negative

A current carrying wire, with its associated magnetic field will experience a “motor effect” if placed (at a non- zero angle) in a magnetic field The force is perpendicular to both the current & the magnetic field

Experimentally, it can be shown that the size of the force due to the motor effect is related to the following : 1.The strength of the current 2.The strength of the magnetic field 3.The length of the wire 4.The angle between the field lines & current In terms of angles, the force is greatest when the current is perpendicular to the field and zero when parallel to the field

The relationship between field, current and force can best be remembered using “Fleming's left hand rule” First finger (Field), seCond Finger (Current), Thumb (moTion)

Expressing the observations as proportionalities : Force  Current (F  I) Force  length of wire (F  l ) (that is a lower case L) F  I l For a given magnetic field we can turn this into an equation where F = BI l (assuming current is perpendicular to field) Where B is the magnetic flux density and is the force per unit length, per unit current (N/m/A) but given the unit of Tesla (T) (Note we can introduce a sin  term to the above equation to consider angle but it is beyond our spec

A straight horizontal wire of length 5m is in a uniform magnetic field which has a magnetic flux density of 120mT. The wire is perpendicular to the field lines which act due North. When the wire conducts a current of 14A from East to West calculate the magnitude and direction of the force on the wire Using F = BI l F = 120 x x 14 x 5 F = 8.4N Vertically downwards Current East to West Field due North

a) 2.4x10 -2 N West, b) 4.5A east to west, c) 0.2T vertically down, d) 8.0x10 -3 N due south